Conductive drilling fluids



United States Patent CONDUCTIVE DRILLHNG FLUIDS Paul W. Fischer, Whittier, Califl, assignor to Union Oil Company of California, Los Angeles, Calrf., a corpora tion of California No Drawing. Application June 16, 1952, Serial No. 293,847

23 Claims. (Cl. 252-85) cally conductive, and to means for conducting electric well logging operations. I

In drilling oil or gas wells by means of rotary drilling tools, a hollow drill pipe having a bit attached to its lower end is extended downwardly through the well bore and is rotated while the bit is pressed against the working r face in the formation at the bottom of: the hole. The action of the rotating bit grinds away the formation as the drilling progresses. During the drilling a fluid body known as a drilling fluid is continuously circulated down through the drill stem, through the bit and against the working face of the hole, and then back up to the surface through the annular space between the drill stem and the walls of the bore hole. The drilling fluid serves a number of purposes, among which are cooling and lubricating the drill bit, suspending and removing cuttings from the bore hole, preventing the flow of liquids from the formation traversed by the bore into the same by exerting a hydrostatic pressure on such formation, and fulfilling other purposes.

In locations where the underground formations traversed and/or penetrated by the bore contain materials such as hydratable clays which swell and/or disintegrate in the presence of water, it has become customary to employ drilling fluids which are relatively free of water, e. g. fluids which contain less than about 10 per cent of water, in order to preclude or minimize the introduction of water into the bore by means of the drilling fluid. Such drilling fluids are termed oil-base fluids since they almost invariably comprise a mineral oil having dispersed or suspended therein minor proportions of various agents adapted to impart the requisite physical properties to the base oil. Among the most important of such agents are: weighting agents, which are high density inert solids adapted to increase the apparent density of the base oil and thus increase the hydrostatic head provided by the drilling fluid within the bore; wall-building agents, which are materials such as clay or asphalt adapted to coat or plaster the walls of the bore with an impermeable layer which prevents escape of the drilling fluid into permeable formations; and dispersing agents, which serve to maintain solid components of the fluid uniformly dispersed therein.

Among the various methods for investigating and determining the nature of the subsurface formations traversed by'a well bore, i. e., well logging, those involving the measurement of one or more of the electrical charac teristics of such formations enjoy wide application. Most of such methods require that the bore hole he filled with a liquid capable of conducting relatively low voltage electric currents, and it is hence highly desirable that the drilling fluid employed in drilling the well be adapted for such use, i. e., the drilling fluid should have relatively good electrical conductivity. In general, how.- ever, oil-base drilling fluids are not sufliciently conductive to adapt them for use during electric logging operations, and this deficiency has greatly restricted their general applicability despite their other highly desirable properties and characteristics. Typically, oil-base drilling fluids have electrical resistivities of the order of 1X10 ohm-ems, which is far too high to permit their use in electric logging operations where it is usually desirable that the bore hole fluid have a resistivity not greater than about 50 10 ohm-ems. and preferably below about 5X10 ohm-cms. 1

Inasmuch as oil-base drilling fluids normally contain a small amount of water, usually about 0.5-5 per' cent by weight of the entire composition, it would appear that their electrical resistivity could be decreased simply by adding to the fluid a water-soluble electrolyte which would dissolve in the Water to form an electrically conductive phase within the body of the fluid. I have found, however, that the addition of electrolytes such as sodium phosphate, sodium chloride, calcium chloride, etc. to oilbase drilling fluids has little effect on the electrical conductivity of the fluid, and, furthermore, often adversely affects other properties of the fluid. For example, the addition of about 2 per cent by weight of sodium phosphate to a soap-stabilized drilling fluid of 'the type described in U. S. Patent 2,542,020 not only fails to reduce electrical resistivity appreciably but also increases the fluid loss value by more than fourfold. In this respect, the oil-base drilling fluids to which the present invention relates differ markedly from the so-called emulsion-base fluids. The latter, whether they be of the oil-in-water or water-in-oil type, contain a minimum of about 10 per cent by weight of water and when they contain about 20 or more per cent of water they can more or less readily be rendered electrically conductive without adversely affecting their fluid loss characteristics simply by dissolving an ionizable salt in the aqueous phase. Oil-base drilling fluids, on the other hand, derive their desirable characteristics from the fact that they contain a minimum amount, e. g. less than about 10 per cent, of .water and in contrast to the emulsion-base fluids, they cannot be rendered conductive simply by the addition of an ionizable salt or other electrolyte. It will accordingly be understood that the term oil-base fluid, as herein employed, refers to drilling fluid compositions in which the base fluid is mineral oil and in which the water content is less than about 10 per cent by weight.

I am aware that certain specific oil-base drilling fluids, i. e. those which contain a mixture of alkali-metal and alkaline-earth metal soaps of heat-treated rosin acids can be rendered conductive by incorporating in the fluid carefully controlled amounts of sodium silicate. However, in order to gain the benefit of the use of sodium silicate in this manner, it is necessary that the amount of alkalineearth metal soap employed and the water content of the fluid be likewise very carefully controlled. For best results it is necessary to employ a homogenizer to insure adequate dispersion of the sodium silicate in the base fluid. As will be apparent to those skilled in the art, such careful control of proportions and unconventional mixing procedure is very diflicult to achieve under field conditions where inadequate mixing facilities render almost impossible the preparation of completely uniform compositions and where the fluid is constantly subject to contamination.

It is accordingly an object of the present invention to provide improved oil-base drilling fluids suitable for use in electric well-logging operations.

Another object is to provide oil-base drilling fluids having good electrical conductivity.

Another object is to provide means for improving the electrical conductivity of oil-base drilling fluids, which means are adapted to use under actual field conditions and do not adversely affect other desirable properties of the drilling fluid to any substantial extent.

A further object is to provide a composition of matter adapted to imparting the property of electrical conductivity to oil-base drilling fluids which are normally substantially non-conductive.

A still further object is to provide an improved method for conducting electric well logging operations.

Other objects will be apparent from the following detailed description of the invention, and various advantages not specifically referred to herein will occur to those skilled in the art upon employment of the invention in practice.

I have now found that the above and related objects may be realized by incorporating an electrolyte and an auxiliary emusifying agent into the oil-base drilling fluid. More particularly, I have found that the electrical resistivity of oil-base drilling fluids may be reduced to values within the range required for electric logging operations by adding to the fluid relatively small amounts each of a water-soluble metallic compound, i. e., an electrolyte, and a humic acid soap. In many instances the normal water content of the oil-base fluid will be sufficient to dissolve the added electrolyte, but in other cases a small amount of water is added for this purpose. In order to retain the desirable general characteristics of oil-base drilling fluids, however, the water-content should be maintained below about 10, preferably between about 4 and about 8, per cent by weight of the entire composition.

The water-soluble ionizable metallic compounds which are employed in accordance with the invention to impart electrical conductivity to oil-base drilling fluids are for the most part water-soluble salts of the alkaliand alkaline-earth metals and alkali-metal hydroxides. In accordance with customary nomenclature the ammonium radical is herein included within the term alkali-metal. Watersoluble salts of the heavy metals are not inoperable insofar as initially imparting electrical conductivity to the fluid is concerned, but because of the tendency of such salts to hydrolyze in aqueous solution to form insoluble hydroxides, their effect on electrical conductivity is not usually permanent. Specific examples of the foregoing class of water-soluble ionizable metal compounds employed in practice of the invention include the hydroxides of sodium, potassium and lithium, sodium chloride, barium chloride, potassium iodide, ammonium chloride, calcium bromide, magnesium chloride, calcium nitrate, barium nitrate, magnesium nitrate, sodium sulfate, potassium sulfate, magnesium sulfate, sodium carbonate, lithium carbonate, potassium bicarbonate, sodium silicate, potassium silicate, sodium phosphate, ammonium phosphate, sodium polyphosphate, potassium bisulfate, sodium sulfite, sodium acetate, potassium arsenate, sodium borate, potassium dischromate, sodium cyanide, calcium nitrite, ammonium hydrogen phosphate, sodium thiosulfate, sodium chromate, sodium thiosulfate, sodium molybdate, potassium phosphite, sodium manganate, etc.

Mixtures of such compounds may likewise be employed. In general, best results are obtained by employing compounds which dissociate in water to form alkaline solutions, i. e., hydroxides and salts of strong bases and weak acids. The alkali-metal hydroxides, silicates, and phosphates are particlarly preferred.

The emulsifying agent which is employed in conjunction with the aforementioned electrolytes in accordance with the invention is the product obtained by reaction between humic acid and an alkali-metal base. Humic acid is the term applied to the alkali-soluble materials formed by the natural decomposition of humus or lignocellulose. These materials are colloidal solids of unknown constitution, and may vary in color from darkbrown to light-tan depending upon their degree of purification. Chemically, they are slightly acidic and are composed of carbon, hydrogen and oxygen with small amounts of sulfur and traces of nitrogen. Commercial humic acid usually contains varying amounts of moisture and ash. A typical analysis, on a moistureand ash-free basis, shows:

Percent by weight Maegan 2,1

Most commercial samples contain moisture and ash in the order of about 15 and about 25 per cent, respectively. The reaction between humic acid and an alkalimetal base takes place readily at atmospheric pressures and temperatures in aqueous media, and may be carried but simply by stirring the humic acid into an aqueous solution of the desired base, e. g., sodium hydroxide, potassium hydroxide, sodium carbonate, ammonium hydroxide. Sodium hydroxide is preferred, and is preferably employed in relatively dilute solution, e. g., 2-10 per cent by weight, in order to avoid undue degradation of the humic acid. The alkali-metal base should be employed in an amount at least suflicient to neutralize the humic acid. Such amount usually represents between about 5 and about 10 per cent by weight of the humic acid depending upon the particular base employed and the neutralization number of the humic acid.

The proportions in which the electrolyte and the emulsifying agent are incorporated in the drilling fluid are to a certain extent interdependent, i. e., with increasing amounts of the metal compound the amount of emulsifying agent may be decreased, and vice versa. Also, certain particular metal salts require the presence of more of the emulsifying agent than others and certain particular humic acid soap products are more effective in a given amount than others. In general, however, the water-soluble metal salt or alkali-metal hydroxide is provided in an amount representing between about 0.01 and about 5 per cent, preferably between about 0.1 and about 2 per cent, by weight of the entire composition, and the emulsifying agent is employed in an amount representing between about 0.1 and about 8 per cent, preferably between about 0.4 and about 4 per cent, by weight of the entire composition.

As previously stated, it is necessary that the drilling fluid contain suflicient water to dissolve the water-soluble metal salt or hydroxide so that the latter becomes dispersed in the fluid in the form of a relatively dilute solution. In many instances the drilling fluid will normally contain suflicient water for this purpose, in which case no water need be added along with the metal salt or hydroxide. In some instances, however, it will be necessary to add a small amount of water. In general, it is desirable that the drilling fluid contain at least about 3 per cent, preferably at least aboutS per cent, by weight of water. The upper limit on the water content of the fluid .is established at about per cent by weight since those fluids which contain substantial amounts of water lose the desirable characteristics of oil-base drilling fluids in general, and are more properly classifiable as emulsion-base fluids.

The use of a water-soluble metal salt or alkali-metal hydroxide in combination with an emulsifying agent of the above-defined class in accordance with the invention is applicable to oilbase drilling fluids in general, regardless of their exact formulation. In general, all oil-base drilling fluids essentially comprise a mineral oil dispersion of solids and a dispersingagent which serves to maintain the solids more or less stably dispersed in the oil. When a hydratable clay is included as a wall-building agent the fluid usually also contains a small quantity of water. Almost invariably the base oil is a mineral oil, and may be crude oil, a distillate, or a residual fraction. Very often blends of distillate and residual fractions are employed, e. g., a blend of a light distillate such as keroseine or Diesel fuel and a light residual fraction such as furnace oil or a light fuel oil. The dispersed solids may serve solely as a weighting agent, in which case they usually take the form of finely-divided inert metallic compounds such as lead dust, barytes, iron oxide, "calcined clay, whiting and the like, or they may serve as wallbuilding agents to coat or plaster the walls of the bore with an impermeable layer which prevents escape of the drilling fluid into permeable formations traversed by the bore. Such wall-building agent usually comprises a hydratable clay such as bentonite, in which case a small amount of water is included in the fluid for the purpose of effecting hydration of the clay. Asphalt is also employed as a wall-building agent. In many instances the dispersed solids may comprise both a weighting agent and a wall-building agent. A wide variety of materials may be employed as dispersing or suspending agents to maintain the solids uniformly dispersed in the base oil. For the most part, however, the dispersing agent will comprise a metal soap of a fatty, resin or naphthenic acid. In some instances such soaps are formed in situ by incorporating into the fluid a soap-forming acid, such as tall oil, rosin, oleic acid, sulfonic acid, linoleic acid, linseed acids, and the like and a basic inorganic compound such as sodium hydroxide, lime, or sodium silicate. In other cases the soap may be formed in situ by incorporating an alkali-metal soap and an alkaline earth metal base into the fluid, whereby a metathesis reaction occurs to form the corresponding alkaline-earth metal soap. Soap mixtures, including mixtures of water-dispersible and oil-dispersible soaps may also be employed. In addition to the soap-type dispersing agents, such materials as lampblack and diatomaceous earth have been employed for the same purpose.

While the principle of the invention is applicable broadly to all types of oil-base drilling fluids, it is particularly applicable to the soap-stabilized fluids of the type described in U. S. Patent 2,542,020. Such fluids are prepared by dispersing small amounts each of a hydratable clay, an alkaline-earth metal base, and an alkali-metal soap of a heat-treated rosin in a suitable base oil. A partial metathesis reaction occurs between the rosin soap and the alkaline-earth metal base whereby there is obtained a mixture of the corresponding alkaline-earth metal rosin soap and unreacted alkali-metal rosin soap. The alkalimetal rosin soap employed in preparing this type of drilling fluid is obtained by reacting an alkali-metal alkali, e. g. sodium or potassium hydroxide, with heat-treated wood or gum rosin in such a manner that the reaction is only partially complete and the saponified product contains from about 1 to about per cent of free unsaponified resin acids.

The heat-treatment of rosin, whereby the resin acids thereof are isomerized and/ or otherwise modified, is well known in the naval stores art, and may be effected .in

various ways to obtain modified rosin products whichvary somewhat in their physical and chemical properties depending upon the nature and extent of the heat-treatment. Thus, any of the various color grades of refined wood or gum rosin may be heated under non-oxidizing conditions at temperatures between about 250 C. and about 350 C. for a length of time suflicient to raise the specific rotation of the rosin from its original negative value to a value between about +5 and about +15". The resulting rosin product closely resembles the original rosin in appearance, ease of saponification, etc., but is considerably altered chemically as evidenced by its increased specific rotation, increased dehydroabietic acid content, lower iodine number, etc. By carrying out the heat-treatment at somewhat higher temperatures and/or over longer periods of time, the specific rotation maybe raised further, e. g., to +25 or even higher, and the degree of olefinic unsaturation further decreased. Also, under such conditions decarboxylation takes place with the formation of unsaponifiable bodies which are usually referred to as rosin oils. The heat-treatment of rosin to secure the desired modification of the resin acids as indicated by increase in specific rotation to a value above about +5 may also be eifected in the presence of catalysts at relatively low temperatures as described in U. S. Patent 2,154,629. The catalysts employed are of the hydrogenation type, e. g., metallic platinum or palladium, although the treatment is carried out in the absence of added hydrogen. The reaction which takes place is termed disproportionation since it involves the simultaneous hydrogenation and dehydrogenation of abietictype acids with the consequent formation of dihydroabietic and dehydroabietic acids and their analogues, and the resulting product is referred to as disproportionated rosin. Similarly, the product obtained by heat-treating rosin under conditions sufficiently drastic that carboxyl groups are removed from the rosin acids is termed decarboxylated rosin, and the product obtained by heattreating rosin under less drastic conditions so that the change effected is substantially only one of molecular rearrangement is referred to as isomerized rosin. All of these modified rosin products are characterized by having been prepared by heat-treating rosin under conditions of time and temperature, and in the presence or absence of a hydrogenation catalyst but in the absence of added hydrogen, suflicient to raise the specific rotation of the rosin to a value above about +5".

Any of the above described modified rosin products may be used to obtain the saponification products employed in preparing the preferred drilling fluid compositions of the present invention. Procedure for carrying out the saponification reaction is well known in the art, and in general consists merely of adding the modified rosin in the solid or molten state to a hot aqueous solution of the desired alkali-metal alkali and thereafter heating the mixture until the reaction is complete and the product contains the desired amount of water. The amount of alkali employed is somewhat less than that required for the complete saponification of the resin acids in order that the saponification product may contain the requisite amount of free unsaponified resin acids. The concentration of the aqueous alkali is usually so adjusted that the product obtained takes the form of a viscous liquid or thick paste containing 60-85 per cent solids. The physical form of the product also depends somewhat upon the type of modified rosin employed. The saponification product obtained from decarboxylated wood rosin containing a substantial amount of rosin oils, for example, is a relatively fluid liquid even though it may contain only 5-10 per cent of water.

While any of the alkali-metal alkali saponification products of rosin which has been heat-treated to raise its specific rotation to a value above about +5 may be employed in preparing the oil-base drilling fluids, I have found that superior results, particularly with respect to the fluid loss value of the drilling fluid, are attained by employing either of two specific products of this type. The first of such preferred saponification products is an alkalimetal alkali saponification product of rosin which has been heat-treated at temperatures between about 250 C. and about 350 C. in the absence of a catalyst to such an extentthat it contains only about 50-60 per cent of free resin acids, 30-40 per cent of unsaponifiable oils, and small amounts of phenolic materials, water, and products of unknown constitution. A particularly preferred prodnet of this type is the potassium hydroxide saponification product of such heat-treated rosin containing about 45-55 per cent potassium resin acid soaps, about 30-35 per cent unsaponifiable materials, about -10 per cent free resin acids, and about 5-10 per cent water. The second of the preferred class of saponification products is the product obtained byheating rosin at a temperature of about 225 300 C. for about -60 minutes-in contact with a hydrogenation catalyst but in the absence of added hydrogen, distilling the resulting product and collecting a fraction distilling at about 210-275 C. under about 5-10 mm. pressure, and thereafter saponifying such fraction with aqueous sodium hydroxide in the known manner. Such product is available commercially under the trade name Dresinate 731. Mixtures of these two types of saponifled heat-treated rosin products may also be employed.

The proportions in which the components of this type of drilling fluid are employed may be varied between certain limits depending on the identity of such components and the specific properties desired in the composition. Ordinarily, however, the saponified heat-treated rosin product is employed in an amount representing between about 1 and about 10, preferably between about 4 and about 8, per cent by weight of the entire composition. The alkaline-earth metal base, which is preferably calcium hydroxide or calcium oxide, is employed in an amount corresponding approximately to that chemically equivalent to the saponified rosin product. When the latter is one of the preferred products hereinbefore described and the alkaline-earth metal base is calcium oxide or hydroxide, the saponification product is provided in the above-mentioned amount and the base is employed in an amount representing between about 0.1 and about 5, preferably between about 0.4 and about 2, per cent by weight of the entire composition. The hydratable clay is employed in an amount representing between about 0.1 and about 5, preferably between about 0.4 and about 1.2, per cent by weight of the entire composition, and the water is provided in an amount representing between about 0.2 and about 10, preferably between about 1 and about 5, per cent by weight of the entire composition. These proportions of water include any water which may be contained in the saponified rosin product and/ or other components, and accordingly the amount of water actually added during preparation of the composition will be adjusted according to the water content of the other components so that the final composition will contain water in the above-mentioned proportions.

In determining the electrical resistivity of the drilling fluids provided by the invention, an electrode assembly comprising two l-inch square nickel plates spaced about one inch apart is immersed in a sample of the fluid being tested, and the voltage which must be applied across the electrodes to obtain a predetermined current flow through the fluid (usually 400 milliarnperes) is ascertained. By calibrating the electrode assembly against a liquid of known resistivity, the resistivity of the fluid sample being tested may be determined from such voltage reading. It has been found, however, that many drilling fluids undergo a dielectric breakdown during such testing procedure. Thus, when the fluid is first subjected to the test voltage across the electrodes may be increased to a relatively high value, e. g., 130 volts, before substantial ourrent flows. As soon as the current starts to flow, however, the voltage may be substantially reduced without reducing the flow of current below the predetermined value. Accordingly, in making the resistivity determination, the electrode assembly is immersed in the fluid and the voltage applied to the electrode plates is gradually raised until the predetermined flow of current is obtained. A so-called initial resistivity value is determined from the applied voltage. The voltage is then gradually reduced, and the minimum voltage required to maintain the predetermined current flow is ascertained, and a so-called ultimate resistivity value is determined from such minimum voltage reading. The dielectric breakdown of the fluid is more or less permanent and the ultimate resistivity value represents the resistivity which the fluid will have during electric logging operations.

The following examples will illustrate a number of ways in which the principle of the invention may be applied, but are not to be construed as limiting the same.

EXAMPLE I The following concentrate composition is prepared:

Parts by weight Diesel fuel 2800 Saponified decarboxylated rosin 1400 Saponified disproportionated rosin 1400 Bentonite 840 Water 560 The diesel fuel is a light domestic diesel oil having an API gravity of about 31, a viscosity of about 40 SUS at F. and a boiling range of about 400-720 F. The saponified decarboxylated rosin contains 45-55 per cent of potassium resin acid soaps, 30-55 per cent of unsaponifiable rosin oils, 5-10 per cent of free resin acids and 5-10 per cent of water. The saponified disproportionated rosin is the hereinabove described Dresinate 731.

This concentrate composition is then partially diluted with a further quantity of the diesel fuel in a ratio of 3 gallons of diesel fuel per 2 gallons of concentrate, and the partially diluted concentrate is then further diluted with fuel oil in a ratio of 17 gallons of fuel oil per 5 gallons of the partially diluted concentrate. Approximately 740 parts by weight of calcium oxide are then stirred into the completely diluted concentrate. The fuel oil is a light domestic fuel oil having an API gravity of about a viscosity of about 36 SSF at 127 F. and a flash point of F.

The finished drilling fluid has the following approximate composition:

Percent by weight Domestic diesel fuel 15.7 Light domestic fuel oil 77.7 Saponified decarboxylated rosin 1.9 Saponified disproportionated rosin 1.9 Calcium oxide 0.9 Water 0.8 Bentonite 1 1 The following Table I presents data illustrating the effect of adding various electrolytes in combination with water and an emulsifying agent of the present class to the above drilling fluid. In each experiment, the indicated proportions of the electrolyte, water and emulsifying agent are stirred into a 1500 ml. sample of the drilling fluid, and the electrical resistivity of the resulting composition is determined as hereinbefore described. The emulsifying agent is a commercial product (cold water-soluble grade) obtained by reacting purified humic acid with a chemically equivalent amount of sodium hydroxide.

Table I Resistivity Ohm- Electrolyte Emulsi Expt. Water, tying cms'xma No. Percent 1 Agent,

Identity Percent 1 Percent Initial Ultimate 0.0 0. 0.0 1, 000 1, 000 0. 0 6.2 0. 0 1, 000 1, 000 0. 0 6. 2 1. 2 1, 000 1, 000 0. 0 6. 1 2. 4 1, 000 1, 000 0.0 6. 0 4.8 1, 000 1,000 0. 0 5. 7 9. 1 120 60 0. 6 6. 0 2. 4 4 4 4.4 0. 6 6. 0 2. 4 38 2. 1 0. 6 6.0 2. 4 7. 5 2. 1 0. 6 6. 0 2. 4 55 3. 2 11- gogium gicglrboniatedbu 0. 6 6. 0 2. 4 50 4.4

o rum y 0X1 e, {Sodium silicate, 0.9% i 5 0 4 3 2 0 Trisodiurn phosphate, 0.9%

Sodium hydroxide, 0.6%. 2 4 6 0 2. 4 2 1 1.0 pdiun silicafite, 059%..

rrso ium p osp a e, {Calcium chloride, 0.07%.--. 97 6 0 4 33 2 Trisodium phosphate, 0.9%. 15 Sodium hydroxide, 0.6'7 1 6 6.0 2.4 11 4. 4

' Calcium chloride, 0.07%, r

1 Based on weight of entire composition.

EXAMPLE H Approximately 900 parts of tall oil, 450 parts of sodium silicate (N grade), 1500 parts of air-blown asphalt and 450 parts of sodium hydroxide are stirred into about 5000 parts of light diesel fuel, and the resulting composition is diluted with about 5400 parts of light domestic fuel oil. The resulting oil-base drilling fluid is stirred for one hour, after which it is divided into 1500- part portions. To each portion there is then added 100 parts of water and 60 parts of sodium humate. Approximately 15 parts of the salt indicated in the following table II are then added to each of the portions of the drilling fluid, andthe resistivities of the resulting compositions are determined as previously described.

1 A sodium ethylene diamine tetra-acetate.

EXAMPLE III Approximately 4000 parts of a commercial drilling fluid concentrate comprising asphalt and lime are stirred into 9800 parts of light diesel fuel. The resulting drilling'fluid is divided into 1500-part portions, and to each portion is added 100 parts of water, 60 parts of sodium humate and 10 parts of the salt indicated in the following Table III. The resistivity of each sample is indicated in the table:

Table III I Resistivity, Ohn1-cms.Xl0

Expt. No. Salt Initial Ultimate None 1, 000 1', 000 Sodium bicarbonate 13 Trisodium phosphate 22 8. 5 25 Trisodium polyphosphate. 8 1.0

of the water-soluble salt.

The foregoing examples illustrate the use of various electrolytes anda humic acid soap for'imparting electrical conductivity to several difierent types of freshlyprepared oil-base drilling fluids. In many instances, however, it is desirable to treat drilling fluids which have previously been used in the field. Such field fluids are almost invariably contaminated with calcium and/ or sodium compounds, and while the principle of the present invention isbroadly applicable to such drilling fluids, I have found that superior results are attained when the electrolyte comprises a mixture of an alkali-metal hydroxide and a water-soluble salt of a strong base and a weak acid, e. g., sodium silicate, sodium carbonate, sodium phosphate, etc. Such mixed electrolyte and the humic acid soap may be added separately to the drilling fluid, but a more convenient procedure consists in preparing a pre-mixed composition which may be packaged and sold as such as a conductivity additive to be admixed with the drilling fluid whenever desired. Such composition suitablycomprises 100 parts by weight of water, between about 10 and about 40 parts 'by weight of a mixture of an alkali-metal hydroxide and a water-soluble salt of a strong base and a weak acid, and between about 20 and about parts by Weight of the humic acid soap. The electrolyte mixture will comprise from about 10 to about per cent by weight of the alkalimetal hydroxide and from about 90 to about 10 per cent by weight In employing such a conductivity additive" composition to impart electrical conduc- 'tivity to an oil-base drilling fluid, the composition is simply stirred into the fluid in an amount equal to between about Sand about 20 per cent by weight of the en- 'tire composition.

The use of such conductivity additive composition is of course not limited to contaminated field fluids but is also applicable to drilling fluids which have not previously been used.

The following example illustrates the use of a mixed electrolyte in conjunction with a typical field fluid as above described:

EXAMPLE iv Table IV Resistivity, Ohmems. 10 Expt. No. Electrolyte Initial Ultimate 26 None 500 75 27 Trisodium phosphate, 9 parts.-. 500 38 28 {Trisodium phosphate, 3 parts... 2 2 Sodium hydroxide, 5 parts 29 {Trisodium phosphate, 9 parts.. 4 4 1 Sodium hydroxide, parts...

The following examples are illustrative of typical conductivity additive compositions within the scope of the invention:

EXAMPLEV Parts Sodium humate 35 Sodium hydroxide 3 Sodium silicate 10 Trisodium phosphate 10 Water 50 This composition is added. to the drilling fluid in an amount representing about 10 per cent by weight of the entire composition.

Sodium hydroxide 5 Trisodium phosphate Sodium silicate 10 Water 100 Isopropyl alcohol 10 The foregoing Example VII ilustrates the use of the present humic acid soap emulsifying agent in combination with an alkyl aryl sulfonate dispersing agent. In my copending application Serial No. 284,451, filed April 25, 1952, now U. S. Patent No. 2,696,468, there are disclosed conductive oil-base drilling fluids of the present general type in which an alkyl aryl sulfonate dispersing agent is employed as the auxiliary emulsifying agent. I have found that advantageous results by way of lower cost and improved fluid loss characteristicsmay be attained by replacing a portion of the relatively expensive alkyl aryl sulfonate dispersing agent with a relatively cheap humic acid soap. Accordingly, the present invention also comprises conductive drilling fluids in which the auxiliary emulsifying agent comprises a mixture of an alkyl aryl sulfonate dispersing agent and a humic acid soap. A wide variety of alkyl aryl sulfonate dispersing agents are known, any of which may be employed in combination with the humic acid soap. Typical ex amples includes sodium isopropylnaphthalene sulfonate, potassium dodecylbenzene sulfonate, ammonium tetrahydronaphthalene sulfonate, monoethylphenylphenol sulfonate, sodium cetylnaphthalene sulfonate, etc. Many of such products are commercially available under such trade names as Santomerse, Oronite, Daxad, Aerosol OS, Alkanol, Nacconol, etc. Since both the humic acid soap and alkyl aryl sulfonate dispersing agent are operable in the absence of the other, the relative proportions in which the two components are employed in the mixture may be varied between very wide limits. Usually, however, it is preferred to employ between about 25 and about 75 per cent by weight of the alkyl aryl sulfonate dispersing agent and between about 75 and about 25 per cent by weight of the alkali-metal humate. If desired, a small amount of a solubilizing agent, such as isopropyl alcohol, may be employed to promote solubility of the sulfonate in the oil-base drilling fluid.

The following example illustrates the use of a mixture of an alkyl aryl sulfonate and a humic acid soap in preparing conductive oil-base drilling fluids in accordance with the invention.

EXAMPLE VIII To each of a number of 1500-part portions of the drilling fluid prepared in Example I, there is added 100 parts of water, 40 parts of sodium humate, 40 parts of sodium dodecylbenzene sulfonate (Oronite), 15 parts of isopropyl alcohol, and the amount of electrolyte given in the following table. The resistivities of the compositions so prepared are as follows:

Resistivity, Ohm-emsXlO Expt. No. Electrolyte Initial Ultimate 30 Trisodium phosphate, 14 parts- 94 11 31 Trisodium phosphate, 14 parts. 4 4 1 0 Sodium hydroxide, 10 parts Trisodium phosphate, 14 parts.-. 32 Sodium hydroxide, 10 parts. 2. 2 1.0

gogium giligateig pltrtsntfl o rum y rox e, par s- {Sodium silicate, 15 parts- 4 Sodium silicate, 15 parts 12 {Trisodium phosphate, 14 parts-.- 12 4 4 Sodium silicate, 15 parts Sodium hydroxide, 10 parts. 68 5. 2 {Sodium hydroxide, 10 parts. 22 3 2 Calcium chloride, 2.5 parts" 'Irisodium phosphate, 3 parts. Sodium hydroxide, 10 parts 3. 2 1. 0 Sodium chloride, 3 parts 39 Calcium chloride, 1.2 parts 78 10 As will be apparent to those skilled in the art, many variations in the composition of the drilling fluid and/or the conductivity additives may be made without departing from the scope of the invention. The essence of the invention lies in adding to a normally non-conductive oilbase drilling mud between about 0.01 and about 5 per cent of a water-soluble metal salt or hydroxide, between about 0.1 and about 0.8 per cent of an alkali-metal soap of humic acid and sufficient water to adjust the total water content of the composition to between about 3 and about 10 per cent, all of said proportions being based on the weight of the entire composition.

The conductive drilling fluids provided by the invention may be employed in any of the various well logging methods which require that one or more electrodes be positioned within a well bore filled with a conductive fluid. Certain of such methods comprise a determination of the electrical resistivity of the earth formations traversed by the bore. Others comprise measuring the so-called selfpotential of such formation. Regardless of the exact nature of the logging method, however, the herein described drilling fluids are well adapted to use as the conductive fluid with which the bore is filled and within which an electrode is submerged.

In the appended claims the term conductivity additive composition is employed to designate a composition of matter capable of imparting substantial electrical conductivity to normally non-conductive oil-base drilling fluids when admixed therewith in relatively small amounts.

Other modes of applying the principle of my invention may be employed instead of those explained, change being made as regards the methods or materials employed provided the compositions stated by any of the following claims, or the equivalent of such stated compositions, be obtained.

I, therefore, particularly point out and distinctly claim as my invention:

1. A conductivity additive composition for oil-base drilling fluids, comprising 100 parts by weight of water, between about 10 and about 40 parts by weight of a mixture of an alkali-metal hydroxide and water-soluble salt of a strong base and a weak acid, and between about 20 and about 80 parts by weight of an alkali-metal soap of humic acid.

2. A conductivity additive composition for oil-base drilling fluids, comprising 100 parts by weight of water, between about 10 and about 40 parts by weight of a mixture of an alkali-metal hydroxide and water-soluble salt of a strong base and a weak acid, and between about 20 and about 80 parts by weight of a mixture comprising between about 25 and about 75 per cent by weight of an alkali-metal soap of humic acid and between about 75 and about 25 per cent by weight of an alkyl aryl sulfonate dispersing agent.

3. A composition according to claim 2 wherein the alkali-metal hydroxide is sodium hydroxide and the watersoluble salt of a strong base and a weak acid is selected from the class consisting of trisodium phosphate, sodium silicate and mixtures thereof.

4. An oil-base drilling fluid comprising a mineral oil carrying suspended solids and suificient of a dispersing agent to maintain said solids dispersed in said oil, between about 3 and about 10 per cent by weight of water, between about 0.01 and about per cent by weight of an electrolyte selected from the class consisting of water-soluble metal salts and water-soluble metal hydroxides, and between about 0.1 and about 8 per cent by Weight of an alkali-metal soap of humic acid.

5. An oil-base drilling fluid according to claim 4 wherein the electrolyte is an alkali-metal hydroxide.

6. An oil-base drilling fluid according to claim wherein the electrolyte is an alkali-metal phosphate.

7. An oil-base drilling fluid according to claim wherein the electrolyte is an alkali-metal silicate.

8. An oil-base drilling fluid according to claim wherein the dispersing agent is a metal soap.

9. An oil-base drilling fluid according to claim 8 wherein the dispersing agent comprises a mixture of water-dispersible and oil-dispersible metal resin soaps.

10. An oil-base drilling fluid comprising a mineral oil carrying suspended solids and suflicient of a dispersing agent to maintain said solids dispersed in said oil, between about 3 and about per cent by weight of water, between about 0.01 and about 5 per cent by weight of an electrolyte selected from the class consisting of water-soluble metal salts and water-soluble metal hydroxides, and between about 0.1 and about 8 per cent by weight of a mixture comprising between about 25 and about 75 per cent by weight of an alkali-metal soap of humic acid and between about 75 and about 25 per cent by weight of an alkyl aryl sulfonate dispersing agent.

11. An oil-base drilling fluid comprising a mineral oil having suspended therein a solid wall-building agent and a finely divided inert weighting agent and suflicient of a dispersing agent to maintain said solids dispersed in said oil, between about 3 and about 10 per cent by weight of water, between about 0.01 and about 5 per cent by weight of an electrolyte selected from the class consisting of water-soluble metal salts and water-soluble metal hydroxides, and between about 0.1 and about 8 per cent by weight of a mixture comprising between about 25 and about 75 per cent by weight of an alkali-metal soap of humic acid and between about 75 and about 25 per cent by weight of an alkyl aryl sulfonate dispersing agent.

12. An oil-base drilling fluid according to claim wherein the electrolyte is an alkali-metal hydroxide.

13. An oil-base drilling fluid according to claim wherein the electrolyte is an alkali-metal phosphate.

14. An oil-base drilling fluid according to claim wherein the electrolyte is an alkali-metal silicate.

15. An'oil-base drilling fluid according to claim wherein said wall-building agent is a hydratable clay.

16. An oil-base drilling fluid according to claim wherein the dispersing agent is a metal soap.

17. An oil-base drilling fluid according to claim 11 wherein the mixture of humic acid soap and alkyl aryl sulfonate comprises from about 25 to about 75 per cent by weight of sodium humate and from about 75 to about 25 per cent by weight of sodium dodecylbenzene sulfonate.

18. An electrically conductive oil-base drilling fluid comprising (1) a normally non-conductive drilling fluid prepared by dispersing in a mineral oil between about 1 and about 10 per cent by weight of an alkali-metal alkali saponification product of rosin which has been heattreated at a temperature between about 250 and about 350 C. for a period of time suflicient to raise its specific rotation to a value above about +5 said saponified rosin product containing between about 1 and about 15 per cent by weight of free resin acids; between about 0.1 and about 5 per cent by weight of a hydratable clay; between about 0.2 and about 10 per cent by weight of water; and an amount of an alkaline-earth metal base corresponding approximately to that chemically equivalent to said saponified rosin product; (2) between about 0.01 and about 5 per cent, based on the weight of the entire composition, of an electrolyte selected from the class consisting of water-soluble metal salts and water-soluble metal hydroxides; (3) between about 0.1 and 0.8 per cent, based on the weight of the entire composition of an emulsifying agent essentially comprising an alkali-metal soap of humic acid; and (4) sufficient water to adjust the water content of the entire composition to between about 3 and about 10 per cent by weight.

19. An electrically conductive oil-base drilling fluid according to claim 18 wherein the electrolyte is provided in an amount representing between about 0.1 and about 2 per cent by weight of the entire composition, the humic acid soap is provided in an amount representing between about 0.4 and about 4 per cent by weight of the entire composition, and suflicient water is provided to adjust the water content of the entire composition to between about 5 and about 10 per cent by weight.

20. An electrically conductive oil-base drilling fluid according to claim 18 wherein the emulsifying agent is a mixture comprising between about 25 and about 75 per cent by weight, of an alkali-metal soap of humic acid and between about 75 and about 25 per cent by weight of an alkyl aryl sulfonate dispersing agent.

21. An electrically conductive oil-base drilling fluid according to claim 18 wherein the alkali-metal alkali saponification product is the potassium hydroxide saponification product of rosin which has been heated at a temperature between about 200 and about 300 C. for a period of time suflicient to raise its specific rotation to a value above about +5 and comprises between about 45 and 55 per cent of potassium resin soaps, between about 30 and about 35 per cent of unsaponifiable materials, between about 5 and about 10 per cent of free resin acids, and between about 5 and about 10 per cent of water.

22. An electrically conductive oil-base drilling fluid according to claim 18 wherein the alkali-metal alkali saponification product is that obtained by heating rosin at a temperature between about 225 and about 300 C. for from about 15 to about 60 minutes in the presence of a hydrogenation catalyst but in the absence of added hydrogen, distilling the resulting product and collecting 15 a fraction distilling between about 210 and about 275 C. under 5-10 mm. pressure, and thereafter saponifying such fraction with aqueous sodium hydroxide.

23. In a well logging method wherein. at least one electrode is positioned within a well bore filled with an electrically conductive fluid, the improvement which consists in employing as said fluid the conductive oil-base drilling fluid defined by claim 4.

References Cited in the file of this patent UNITED STATES PATENTS Moore Oct. 8, 1940 Fischer et a1 May 15, 1951 Fischer Nov. 6, 1951 FOREIGN PATENTS Canada Aug. 22, 1950 

1. A CONDUCTIVITY ADDITIVE COMPOSITION FOR OIL-BASE DRILLING FLUIDS, COMPRISING 100 PARTS BY WEIGHT OF WATER, BETWEEN ABOUT 10 AND ABOUT 40 PARTS BY WEIGHT OF A MIXTURE OF AN ALKALI-METAL HYDROXIDE AND WATER-SOLUBLE SALT OF A STRONG BASE AND A WEAK ACID, AND BETWEEN ABOUT 20 AND ABOUT 80 PARTS BY WEIGHT OF AN ALKALI-METAL SOAP OF HUMIC ACID. 